A firm finger tap to the plastic box cover causes the piezoelectric wafer to produce multiple voltage pulses.  The pulses are fed to a low power voltage comparator.  The comparator converts the low voltage pulses into 9v pulses.  Those pulses are fed to the clock input of one half of a CD4013 dual D flip/flop.  One section is configured as a half second one shot.  The second half is wired as a standard flip flop.  With this circuit, the flip/flop changes state with each finger tap to the box, holding the circuit.  The output of the flip/flop is connected to a dual FET push-pull driver circuit, which feeds power to a pair of capacitors.  The capacitors are wired in series with parallel diodes.  This forms a 235uF non-polarized capacitor.  With this charge pump configuration, the power pulses from the FETs feed either a positive pulse to the relay coil, to latch it, or a negative pulse to unlatch it.  The pulse lasts long enough to fully open or close the relay contacts.